How can I force close TcpListener

Question

I have a service that communicates via tcpListener. The problem is when the user restarts the service - the exception "Address is already in use" is thrown, and the service cannot be started for a couple of minutes or so.

Is there a way to tell the system to stop the old connection so that I can open a new one? (I can’t just use random ports because there is no way for the service to notify clients what the port is, so we must depend on the predefined port)

Answer 1

Set the SO_REUSEADDR socket SO_REUSEADDR before binding to the listening port. It seems that the corresponding .NET code looks something like this:

 SetSocketOption(SocketOptionLevel.Socket, SocketOptionName.ReuseAddress, 1); 

Answer 2

There is a reason that sockets are not used for some time after they are closed. The socket consists of 4 tuples: Source and Dest Port, Source and Dest IP.

Let's say you close the socket a lot and the client was busy sending data to the server. You wait 5 seconds and reopen the server with the same port, and the same client sends data to the same 4 tuples, the server will receive packets with incorrect tcp sequence numbers, and the connections will be reset.

You shoot yourself in the foot :)

This is why the connections have the time_wait status for 2-4 minutes (depending on the distribution) until they are reused. To be clear, I'm talking about SOCKET, not just the tcp listening port.